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Cell
Cell is the structural and smallest unit that can be called alive. A cell is the building block for multicellular organisms and is the only structure an unicellular organism can have. Usually cells are very small and thus can only be seen with the aid of a microscope. They are studied in microbiology. Cells are in the third level of organization. These levels are: *Atoms and molecules *Organic and complex molecules like nucleic acids and proteins *Cell Characteristics #The ability to replicate itself #The need for enzymes for its processes #A membrane that separates the cell so it can have a different chemical environment Parts of the cell There are three main parts on the cell. *The cell membrane: which differentiates it from its environment. *The citoplasm: contains the organeles *The nucleus: contains genetic information For all the parts, check parts of the cell. Origin The origin of cells is intertwined with the origin of life. 3.500 millions of years ago, aproximately, life appeared on Earth. This provable hypothesis was thought by A. I. Oparin. According to him, life on Earth was preceded by a long period in which complex molecules were formed and were being accumulated, process known as chemical evolution. There were four gases on this planet at that time: *Hydrogen (H2) *Water vapor (H2O) *Metane (CH4) *Ammoniac (NH3) Besides this matter, there were great amounts of energy on this young Earth. There were radiactive elements coming from the Earth's core thorough volcanic eruptions. There was heat energy to boil as to cook (humid and dry heat). The water cycle already existed. There were also strong storms with lightning which gave electric energy. And the sun bombed Earth's surface with high energy particles and also some ultraviolet radiation. So, Oparin formulated the hypothesis that organic molecules could be formed from atmosphere gases and they could concentrate in a delicate "soup", on lakes and seas. In the absence of oxygen (anaerobic) these organic molecules could stay untouched. Oparin then published this work on 1992, but most scientists ignored his ideas. It was until 1950 that Stanley Miller made an experiment to test the hypothesis. These experiments have showed that complex organic molecules, such as aminoacids and nucleotids, can arise from inorganic matter under conditions similar to those found on the young Earth. Type of organisms (and cells) According to the means organisms (unicellular and pluricellular) use to get their energy, they are classified as: *Autotroph: the cells and their corresponding organisms make their food themselves. They do not need organic molecules as energy source. There are two varieties: photosynthetic (using light as energy) like certain plants; and quimiosynthetic (using inorganic reactions as energy) like certain bacteria. *Heterotroph: the cells and organisms of this type need a foreign source of organic molecules for energy and building blocks. These are fungi, some microorganisms and animals. *Mixotroph: the organisms of this type will be able to create their own food, but they can also consume other organisms. Cell theory *Every organism is made of one or more cells. *Chemical reactions are done within cells. *Every cell comes from another. *Cells contain heredity information for the organisms they form and they transmit it to the offspring Nucleus classification According to the nucleus, cells are classified as: *Prokaryotes: These cells do not have a differentiated cell and the genetic material is located at the citoplasm, in the form of a circular DNA, called chromosome. *Eukaryotes: These cells have a differentiated nucleus. The genetic material is located at the nucleus also in the way of chromosomes but it is not circular, it is a double helix. Classification There are five kingdoms: *Monera *Protista *Fungi *Plant *Animal (See each article for more information) See also *Microbiology *Prokaryote *Eukaryote *Autotroph *Heterotroph